Gas turbine engines, such as those which power aircraft and industrial equipment, employ a compressor to compress air that is drawn into the engine and a turbine to capture energy associated with the combustion of a fuel-air mixture. One or more fluids are typically circulated throughout the engine. For example, oil may be supplied to one or more bearings in order to clean, cool, and lubricate the bearings.
Referring to FIG. 2A, the fluids are typically conveyed from a fluid source (e.g., an oil tank) 202 to the intended destination (e.g., the bearings or an associated bearing compartment) 206 by a supply tube 210. The fluid is then returned from the destination 206 to the source 202 by a return tube 214. In this manner, a closed-loop system 200 is established. There may be other components included; the system 200 is simplified for the sake of explanation and illustrative convenience. These other components may include additional tubes beyond the tubes 210 and 214.
Referring to FIG. 2B, in order to enhance reliability and avoid a leak impacting the performance/operability of the engine, the tubes (e.g., the tube 210 or the tube 214) may be manufactured as a double walled tube 220, where the fluid is intended to be conveyed by a first tube 232. A second tube 236 serves to contain any fluid that may leak from the first tube 232. The double walled arrangement 220 shown in FIG. 2B is frequently referred to as a “tube within a tube” as the tube 236 has a larger dimension/diameter than the tube 232 and the tube 232 is contained/nested within the tube 236. In this respect, the tube 232 is an inner tube relative to the outer tube 236.
Referring to FIGS. 2C-2D, a system assembly 248 incorporating a double walled tube 220 attached to a three-flanged fitting 254 is shown (in FIG. 2C, an attachment of the double walled tube 220 to an angled, threaded fitting 258 is shown for completeness, the details of which are not pertinent to the instant disclosure), where the fitting 254 itself is formed/fabricated as a unitary piece. At an interface 262 between the inner tube 232 and the fitting 254 a brazing operation may be performed and at an interface 266 between the outer tube 236 and the fitting 254 a welding operation may be performed. Brazing may be used at the interface 262 to eliminate/minimize the likelihood that a leak path may develop. Also, the use of brazing at the interface 262 may help to keep the inner tube 232 stationary within a seat of the fitting 254.
Brazing includes strict requirements in terms of coverage and voids. Inspection of a brazed joint can be difficult or time-consuming as x-rays of the joint may be needed. Other difficulties of brazing include the possible rework of the assembly if excess braze is present at the interface 262 (or any other sealing surface). Materials used in brazing include silver braze or gold nickel braze; these materials represent an added cost to the overall construction of the assembly 248. Furthermore, a preparatory step of nickel-flashing (which may include an electrodeposition process) the inner tube 232 and the fitting 254 is performed before the brazing occurs, again representing a cost to the construction of the assembly 248.